The relevant background includes the fields of exhaust gas treatment systems and diagnostics therefore. As to the former field of endeavor, there have been a variety of exhaust gas treatment systems developed in the art to minimize emission of undesirable constituent components of engine exhaust gas. It is known to reduce NOx emissions using a selective catalytic reduction (SCR) system, a treatment device that includes a catalyst and a system that is operable to inject material such as ammonia (NH3) into the exhaust gas feedstream ahead of the catalyst. The SCR catalyst is constructed so as to promote the reduction of NOx by NH3 (or other reductant, such as aqueous urea which undergoes decomposition in the exhaust to produce NH3). NH3 or urea selectively combine with NOx to form N2 and H2O in the presence of the SCR catalyst, as described generally in U.S. Patent Application Publication 2007/0271908 entitled “ENGINE EXHAUST EMISSION CONTROL SYSTEM PROVIDING ON-BOARD AMMONIA GENERATION”, the contents of which are incorporated by reference. For diesel engines, for example, selective catalytic reduction (SCR) of NOx with ammonia is perhaps the most selective and active reaction for the removal of NOx in the presence of excess oxygen. The NH3 source must be periodically replenished and the injection of NH3 into the SCR catalyst requires precise control. Overinjection may cause a release of NH3 (“slip”) out of the tailpipe into the atmosphere, while underinjection may result in inadequate emissions reduction (i.e., inadequate NOx conversion to N2 and H2O).
These systems have been amply demonstrated in the stationary catalytic applications. For mobile applications where it is generally not possible (or at least not desirable) to use ammonia directly, urea-water solutions have been proven to be suitable sources of ammonia in the exhaust gas stream. This has made SCR possible for a wide range of vehicle applications.
Increasingly stringent demands for low tail pipe emissions of NOx have been placed on heavy duty diesel powered vehicles. Liquid urea dosing systems with selective catalytic NOx reduction (SCR) technologies have been developed in the art that provide potentially viable solutions for meeting current and future diesel NOx emission standards around the world. Ammonia emissions may also be set by regulation or simply as a matter of quality. For example, proposed future European emission standards (e.g., EU 6) for NH3 slip targets specify 10 ppm average and 30 ppm peak. However, the challenge described above remains, namely, that such treatment systems achieve maximum NOx reduction (i.e., at least meeting NOx emissions criteria) while at the same time maintaining acceptable NH3 emissions, particularly over the service life of the treatment system.
In addition to the substantive emissions standards described above, vehicle-based engine and emission systems typically also require various self-monitoring diagnostics to ensure tailpipe emissions compliance. In this regards, U.S. federal and state on-board diagnostic regulations (e.g., OBDII) require that certain emission-related systems on the vehicle be monitored, and that a vehicle operator be notified if the system is not functioning in a predetermined manner. Automotive vehicle electronics therefore typically include a programmed diagnostic data manager or the like service configured to receive reports from diagnostic algorithms/circuits concerning the operational status of various components or systems and to set/reset various standardized diagnostic trouble codes (DTC) and/or otherwise generate an alert (e.g., MIL). The intent of such diagnostics is to inform the operator when performance of a component and/or system has degraded to a level where emissions performance may be affected and to provide information (e.g., via the DTC) to facilitate remediation.
Over the service life of the above-described exhaust treatment systems, various constituent components can wear, degrade or the like, possibly impairing overall performance. For example, degradation of either the SCR catalyst or the dosing system may impair the treatment system in meeting either or both of the NOx and NH3 emission standards. Diagnostic methods to detect such conditions are described generally in U.S. Patent Application Publication 2010/0101214 entitled “DIAGNOSTIC METHODS FOR SELECTIVE CATALYTIC REDUCTION (SCR) EXHAUST TREATMENT SYSTEMS”, the contents of which are incorporated by reference. However, improvements are always desirable in any art.